Method and composition for the treatment of an aluminum surface



United States Patent 3,349,014 METHUD AND COMPOSITION FOR THE TREAT- MENT OF AN ALUMINUM SURFACE William H. Hyter, Los Angeles, Calif., assignor, by mesne assignments, to McDonnell Douglas Corporation, Santa Monica, Calif, a corporation of Maryland No Drawing. Filed Aug. 28, 1964, Ser. No. 392,953

13 Claims. (Cl. 20415) This invention relates to the anodizing of aluminum parts, and is particularly concerned with a method of and an electrolyte for producing corrosion resistant coatings on aluminum parts.

It is well known in the aircraft industry to provide anodic coatings on aluminum and aluminum alloy parts to render the parts corrosion resistant and also to form a bonding surface for adhesion of paints to the surface of the parts. Such procedure involves complete immersion of the parts in a suitable electrolyte and passage of an electric current through the part with said part connected to form the anode. In the case of large components and assemblies this requires maintenance of large tanks of electrolyte.

Large anodized parts such as aircraft wing panels, frequently receive scratches, tool marks, and like damage during fabrication and assembly, which removes the anodic coating. When a large anodized aluminum part such as an aircraft assembly, e.g., an aircraft wing panel, is mechanically damaged by scratches, gouges or abrasions, it becomes necessary to reanodize such damaged surfaces to meet the design requirements for corrosion protection, fatigue life and appearance of the panels. It has been found too cumbersome and time consuming again to treat the entire part in the usual anodizing electrolyte, e.g., a chromic acid bath, by the conventional procedure, so as to reanodize these damaged surfaces, which are usually of relatively small area. However, it is necessary to apply a new coating to these damaged areas since otherwise such damaged areas are susceptible to corrosion with danger of possible eventual structural failure, and also presents an undesirable appearance due to the removal of the usually gray anodic coating from such damaged areas.

Since various shades of gray anodic coatings are produced on an aluminum part by the conventional chromic acid anodizing process, there exists not only the problem of applying an anodic coating to relatively small damaged areas, such as scratches and gouges, in the surface of a large aluminum part without affecting the large undamaged surfaces, but it has proven a difficult problem to match a particular gray color of the overall anodic coating when a small surface area is recoated. It has been attempted to apply epoxy paints over a damaged surface area of the above type, but it has been difiicult to match the surrounding coated area using such paints, and such procedure has proven unreliable and time consuming, and further, repainting of such damaged areas is often necessary after a relatively short period of time.

His accordingly one object of the invention to provide procedure and compositions for anodizing an aluminum part, particularly limited areas of the part, quickly, efl'lciently and economically, to obtain an anodic coating having properties, e.g., corrosion resistance*,equal or superior to conventional anodic coatings.

Another object of the invention is the provision of procedure and electrolytic baths for anodizing local surface areas of an aluminum part, the surrounding areas of which have been previously anodized, to provide corrosion protection over the surface of such localized area which is substantially equivalent to the corrosion protection provided by the anodic film originally formed over the remaining area of the part, e.g., by a conventional chromic acid anodizing procedure.

A still further object of the invention is the provision of a process and electrolyte particularly designed for anodizing relatively small, e.g., damaged surfaces, of an aluminum part which has been previously anodized, so that the anodic coating applied to the damaged area or areas has an appearance and color similar to the surrounding anodic coating produced in the conventional manner, e.g., by use of a chromic acid bath.

Yet another object is to provide a process and anodizing bath designed to produce a strong corrosion resistant anodic coating on predetermined limited areas of a relatively large aluminum part surface which has been previously anodized, so that the fatigue life of the limited area thus anodized is substantially as great as that of the initially applied anodic coating, and the anodic coating applied to such limited area has no deleterious effect on the original anodic coating.

Other objects and advantages will appear hereinafter.

The above objects of the invention are achieved by applying to a surface of an aluminum part, e.g., a limited area of the part from which a previously applied anodized coating has been removed due to damage thereto, a porous pad containing an electrolyte comprising an aqueous solution of phosphoric acid, acetic acid, and sulfuric acid, passing direct current through the pad, the electrolyte and the aluminum part with said part connected as anode in the circuit, and moving the pad with respect to the part surface while maintaining contact between the pad and said surface, for a period sufiicient to form an anodic coating on the thus treated part surface.

The term aluminum employed throughout the specification and claims is intended to denote substantially pure aluminum or any of its alloys.

It has been found from experience that the abovenoted method for so-called brush anodizing of a local area of an aluminum part to apply an anodic film thereto can be carried out in a matter of seconds, e.g., about 40 seconds, and that such anodizing technique utilizing direct current and particularly the above-noted electrolyte can be controlled to produce various shades of gray color so as to match the particular gray anodic film of the surrounding area of the part. The shade of gray coloration of coating which is deposited according to the invention is dependent chiefly on the voltage applied, as will be pointed out more fully below. Moreover, the anodic coating thus formed is essentially equivalent to that of the original chromic acid anodized coating on the remainder of the part, in terms of corrosion resistance as measured by salt spray tests and fatigue life of the area of the aluminum part thus coated. Also the anodic film thus deposited according to the invention on a reworked area of a previously anodized panel is produced without any adverse or deleterious effect on the surrounding original anodized coating. Further, the equipment including the brush or pad and the cathode arm to which it is connected, is portable and the process can be readily carried outin the field instead of at plants or factories where electrolyte tanks are required to be maintained for the usual anodizing operations.

In carrying out the process of the invention, e.g., for reworking or reanodizing limited surface areas of a large aluminum part, the surrounding surface area of which retains its initial anodic coating, the surface defects in the limited area being reworked are first sanded and polished to remove rough edges. Any suitable polish can be employed for this purpose. Following such polishing the surface being treated is degreased with trichlorethylene or any other suitable degreaser such as Stoddard solvent, and the like.

Following degreasing a tape is preferably placed around the border of the area to be processed and such area is then treated, e.g., as by swabbing with an aqueous sodium hydroxide solution, to clean such surface area, followed by treatment of such area with an acid such as nitric acid to neutralize the alkali. The treated part surface is then rinsed with water and dried.

The. anodic coating is then applied over the thus prepared surface, employing a pad or a brush constructed of any suitable material capable of holding or becoming saturated with the electrolyte. Thus, for example, a cotton pad or other porous material such as sponge rubber may be employed. The pad, e.g., the cotton tip, is preferably covered with nylon or orlon cloth or some othermaterial which functions to hold the pad in place so that it is not in direct contact with the working surface, and also such cover functions as a lubricating medium for the saturated pad, that is, to facilitate movement thereof over the surface of the aluminum part being processed.

The covered pad is mounted at the end of a metal electrode which functions as cathode in the anodizing process. Such cathode can be formed of any suitable material, such as lead, steel or copper, but is preferably formed of lead since this metal is resistant to attack by the electrolyte solution of the invention.

As previously noted, the novel electrolyte solution of the invention which produces improved results in the anodizing method hereof is composed of three essential components, phosphoric acid, acetic acid, and sulfuric acid, in aqueous solution. Generally, these components are employed in the ranges set forth in Table I below:

Although best results are obtained by operating within the ranges of components set forth in the table above, some improved results can be obtained by employing proportions of the above components outside the ranges set forth in Table I.

The electrode which carries the pad saturated with electrolyte is connected as cathode in a direct current circuit, and the surface of the aluminum part or panel to be anodized is connected as anode- The pad is applied to the area of the surface to be anodized and is brushed or swabbed oversuch area. The voltage applied dan vary from about 23 to about 30 volts, preferably about 23 to about 28 volts. It has been found according to the invention that the color of the anodic coating produced varies With the particular voltage applied. Thus, as voltage is increased from about 23 volts up to about 30 volts the color of the anodic coating progressively darkens. To obtain a very light colored anodic coating having a metallic color, a voltage varying from about 23 to about 25 volts is applied, for an intermediate light gray anodic coating, voltage is varied between about 24 and about 26 volts, and for a dark gray anodic coating voltage is made to rangebetween about 26m about 28 volts. Thus, it can be seen that the anodic coating applied to the limited area of the part according to the invention and employing the electrolyte described above can be made to match the surrounding initially applied chromic acid anodized coating by controlling the voltage.

The above-noted voltage ranges apply forcathodes having approximately a inch square surface area. Where larger cathodes are employed voltage applied generally will be increased. Thus, when large cathodes are employed, voltage can be increased up to the order of about 80 volts or more. Hence, the voltage applied can be in the broad range from about to about 80 volts in the invention process.

The current applied is generally less than about 10 amperes, and preferably ranges from about 0.5 to about 5 amperes, most desirably from about 0.5 to about 2.5 amperes. Although it is preferred to employ only direct current, if desired, alternating current superimposed upon direct currenf can be employed. The time required for applying the anodic coating according to the invention can range from as little as 30 seconds up to about 1 /2 minutes and usually varies from about 40 to about 60 seconds. During the anodizing period, it is preferred to provide continuous movement of the pad containing the electrolyte over the area being worked, so as to obtain a uniform anodic coating over sucharea and to prevent local burning. Completion of the anodizing process is determined by making a check of the conductivity of the anodic coating produced. If the anodic coating formed does not conduct an electric current at a potential in the range of about 50 to about 75 volts direct current, this indicates completion of the process and formation of a corrosion resistant anodic coating according to the invention.

Thus the invention process employing the novel electrolyte solution described above provides strong corrosion resistant anodic coatings in a remarkably short period of time of the order of seconds, whereas in order to obtain the same coating employing conventional chromic acidelectrolyte solutions by a brushing or swabbing procedure, substantially longer time periods, e.g., a period of hours, is required. Even when carrying out the anodizing process by immersion of the part in a tank of chromic acid electrolyte, the required anodizing period is from about 30 to about 40 minutes. Further, by employing the electrolyte of the invention in conjunction with the voltage technique describedabove, the color of the anodic coating deposited can be matched with the color of the surrounding anodic coating on the part. Also, no adverse or deleterious effect on the surrounding original chromic acid anodized coat has been observed, nor have any detrimental effects been observed on the physical properties of the aluminum base..Moreover, corrosion tests of the anodic coatings produced according to the invention have shown that such anodic coatings withstand the standard salt spray tests .up to 20 days or more, whereas chromic acid anodized coatings show corrosion after about, 10 days exposure to such salt spray treatment. It is also apparent from the above that the equipment required to carry out the process is small and compact, and is therefore portable and can be carried out in the field for processing parts away from plants and factories.

Various aluminum alloys can be treated by the procedure and electrolyte described above. These include, for

example, 7075, 2024, 6061, 5052, and the like, aluminum I alloys.

The following are examples of practice of the invention:

Example 1 In Table II below are set forth a series of five specific electrolyte solutions which can be employed according to the procedure of the invention to obtain corrosion resistant anodic coatings.

An aircraft wing panel of 7075-T6 Alclad aluminum was provided with an initial chromic acid anodized coating of light gray color, for corrosion protection. The panel had a number of damaged areas of anodic coating which contained scratches, gouges and other imperfections, each r of such damaged areas varying in size from about 2 to about 6 inches square, with anodic coatings removed from such damaged areas.

The damaged areas of the wing panel were each lined off by masking tape from the surrounding areas. The rough portions of the damaged areas were then sanded and polished lightly with a conventional polishing compound such as that marketed as EZ Shine, and the sopolished areas were then cleaned and degreased with trichloroethylene. Further cleaning of the areas to be reworked according to the invention was then carried out by swabbing such areas with a solution of sodium hydroxide followed by neutralization of the alkali by application of 10% nitric acid to the surface. Such surface was then rinsed with water and dried with a clean white cloth.

To produce the anodic coating according to the invention, a lead cathode about 4 inch square was employed, the end of the cathode being covered with a pad of cotton approximately inch thick to hold the electrolyte, and the cotton was covered with nylon cloth to protect the cotton reservoir.

The lead cathode was connected to the negative wire, e.g., of a rectifier, providing a DC voltage, the positive wire of which was connected to the aluminum surface of the wing panel to be anodized. The cotton pad carried by the cathode was immersed in electrolyte Solution A noted above, sufiicient electrolyte being absorbed by the cotton pad to anodize approximately 10 square inches of area without redipping.

The cotton tip was then applied over the prepared surface area and swabbed or brushed back and forth over the area being worked in like manner as a brush would be used. Direct current ranging from about 0.5 up to about 1.0 amp. was used and since the surrounding initially applied chromic acid anodized coating was light gray, a voltage of about 24 to about 26 volts was applied to match such light gray color of the surrounding anodic coating, using even and smooth strokes of the cotton pad over each of the areas being anodized. This procedure was continued for a period of about 60 seconds for each of the damaged areas being treated, after which the thus anodized surface was rinsed with distilled water and a clean white cloth. Completion of the anodizing process and formation of the anodic coat was ascertained by testing the reworked area for electrical conductivity as indicated by a continuity check, nonconductivity of an electric current of about 50 volts through the reworked areas indicating completion of the process.

Corrosion resistance of the damaged areas to which the anodic coating was applied according to the invention procedure and employing the invention electrolyte, specifically Solution A above, was tested by conventional salt spray test. The anodic coatings produced according to the invention procedure described above showed substantially no corrosion at the end of days of salt spray treatment, whereas a conventional chromic acid anodized coating of substantially the same thickness as the anodic coatings formed on the above-noted damaged areas showed corrosion at the end of about 10 days.

Further, the reworked areas to which the anodic coatings according to the invention were applied had an appearance which blended closely in color with the surrounding initially applied light gray anodic coating, and there was no deleterious effect on the surrounding initially applied anodic coating, as result of the application of the anodic coating according to the invention.

Example 3 The procedure of Example 2 above is substantially repeated except employing as the electrolyte applied to the cotton pad, Solution B above, and a wing panel is employed having a dark gray initially applied anodic coating thereon. In this case voltage is increased from about 26 to about 28 volts during the brushing or swabbing operation for anodizing the reworked damaged surface areas in order to match the dark gray color of the surrounding anodic coat.

Example 4 The procedure of Example 2 is substantially repeated, employing in place of Solution A, electrolyte Solutions C, D and E, respectively. Results similar to Example 2 are obtained.

From the foregoing, it is seen that the invention provides a quick and easy procedure employing a novel electrolyte, for reworking or reanodizing damaged surface areas of a previously anodized surface, from which the coating has been removed, e.g., by scratches, gouges and the like, the reanodized coating having high corrosion resistance and good fatigue life comparable to an initially applied conventional chromic acid anodic coating, and which process can be controlled chiefly by voltage technique so that the color of the reanodized coating matches the color of the surrounding anodic coating.

Although the invention procedure has been described above and has particular value mainly for anodizing a damaged surface area of a part having a previously applied anodic coating, it will be recognized that the invention procedure and electrolyte can be applied likewise to the surface of an aluminum part or object which has not been coated previously, for providing an initial oxide coating on such surface.

It will be understood that various adaptations and modifications of the invention can be made without departing from the spirit of the invention, and hence the invention is not to be taken as limited except by the scope of the appended claims.

I claim:

1. The method of anodizing a predetermined surface area of an aluminum part, which comprises applying a liquid electrolyte to a porous pad, said porous pad being positioned on a metal electrode, said electrolyte comprising about 2 to about 12% by weight of phosphoric acid, about 9 to about 30% by weight of acetic acid, and about 7% to about 60% by weight of sulfuric acid, connecting said electrode, said pad and said part in an electric circuit with said part forming the anode, contacting said pad with said surface area of said part, moving said pad over said predetermined surface area of said part and maintaining continuous contact between said pad and said surface area while passing direct current through said circuit, and continuing said movement of said pad and said passage of current for a period sufficient to form an anodic coating on said predetermined surface area of said part.

2. A process as defined in claim 1, wherein the voltage is in the range from about 15 to about volts.

3. A process as defined in claim 1, wherein the voltage is in the range from about 23 to about 30 volts.

4. The method of anodizing a predetermined surface area of an aluminum part, the surrounding surface areas of said part having an anodic oxide coating thereon, which comprises applying a liquid electrolyte to a porous pad, said porous pad being positioned on a metal electrode, said electrolyte comprising about 2% to about 12% by weight of phosphoric acid, about 9% to about 30% by weight of acetic acid, and about 7% to about 60% by weight of sulfuric acid, connecting said electrode, said pad and said part in an electric circuit with said part forming the anode, contacting said pad with said surface area of said part, moving said pad with respect to said predetermined surface area of said part and maintaining continuous contact between said pad and said surface area while passing direct current through said circuit, and continuing said movement of said pad and said passage of current for a period sufiicient to form a corrosion-resistant anodic coating on said predetermined surface area of said part, and adjusting the voltage during said period so that the anodic coating formed on said predetermined surface area substantially matches the color of the anodic oxide coating in said surrounding surface areas.

5. The method of anodizing a predetermined surface area of an aluminum part, which comprises applying a liquid electrolyte to a porous pad, said porous pad being positioned on a metal electrode, said electrolyte comprising about 2% to about 12% by weight of phosphoric acid,

about 9% to about 30% by weight of acetic acid, and

about 7% to about 60% by weight of sulfuric acid, connecting said electrode, said pad and said part in an electric circuit with said part forming the anode, contacting said pad with said surface area of said part, moving said pad with respect to said predetermined surface area of said part and maintaining continuous contact between said pad and said surface area while passing direct current through said circuit at a voltage of about, 23 to about 30 volts and at, an amperage of less than about 10 amps, and continuing said movement of said pad and said passage of current for a period sufiicient to form an anodic coating on said predetermined surface area of said part.

6. The method of anodizing a predetermined surface area of an aluminum part, the surrounding surface areas of said part having an anodic oxide coating thereon, whichcomprises applying a liquid electrolyte to a porous pad, said porous pad being positioned on a metal electrode, said electrolyte comprising about 2% to about 12% by weight of phosphoric acid, about 9% to about 30% by weight of acetic acid, and about 7% to about 60% by weight of sulfuric acid, connecting said electrode, said pad and said part in an electric circuit with said predetermined surface area of said part forming the anode, contacting said pad with said last mentioned surface area of said part, moving said pad over said predetermined surface area and maintaining continuous contact between said pad and said surface area while passing direct current through said circuit at a voltage of about 23 to about 30 volts and at an amperage of less than about 10 amps, and continuing said movement of said pad and said passage of current for a period sufiicient to form a corrosionresistant anodic coating on said predetermined surface area of said part, and adjusting the voltage during said period so that the anodic coating formed on said predetermined surface area substantially matches the color of the anodic oxide coating in said surrounding surface areas.

7. The method of anodizing a predetermined surface area of an aluminum part, the surrounding surface areas of said part having a light colored to dark gray anodic oxide coating thereon, which comprises applying a liquid electrolyte to a porous pad, said porous pad being positioned on a metal electrode, said electrolyte comprising about 2% to about 12% by weight of phosphoric acid, about 9 to about 30% by weight of acetic acid, and about 7% to about 60% by weight of sulfuric acid, connecting said electrode, said pad and said part in an electric circuit with said predetermined surface area of saidpart forming the anode, contacting said pad with said last mentioned surfacearea of said part, moving said pad over said predetermined surface area and maintaining continuous contact between said pad and said surface area while passing a direct current through said circuit at a voltage of about 23 to about 30 volts and at an amperage of less than about 10 amps, and continuing said movement of said pad and said passage of current for a period of about 30 seconds to about 90 seconds, sufiicient to form a corrosion-resistant anodic coating on said predetermined surface area of said part, and adjusting the voltage during said period so that the anodic coating formed on said predetermined surface area substantially matches the color of the anodic oxide coating in said surrounding surface areas.

8. The method of anodizing a predetermined surface area of an aluminum part, the surrounding surface areas of said part having a light colored to dark gray anodic oxide coating thereon, which comprises applying a liquid electrolyte to a porous pad, said porous pad being positioned on a metal electrode, said electrolyte comprising about 4% to about 8% by weight of phosphoric acid, 15 about 15% to about 24% by weight of acetic acid, and

about 8% to about by weight of sulfuric acid, connecting said electrode, said pad and said part in an electric circuit with said predetermined surface area of said part forming the anode, contacting said pad with said last 20 mentioned surface area of said part, moving said pad over said predetermined surface area and maintaining con tinuous contact between said pad and said surface area while passing a direct current through said circuit at a voltage of about 23 to about 28 volts and at an amperage of about 0.5 to about 5 amps, and continuing said movement of said pad and said passage of current for a period of about 30 seconds to about 90 seconds, sufiicient to form a corrosion-resistant anodic coating on said predetermined surface area of said part, and adjusting the voltage during 30 said period so that the anodic coating formed on said predetermined surface area substantially matches the color of the anodic oxide coating in said surrounding surface areas.

9. The process as defined in claim 8, said voltage being adjusted so that the anodic coating formed on said pre- 30 determined surface area is a light colored anodic coating.

10. The process as defined in claim 8, said voltage being adjusted sothat the anodic coating formed on said predetermined surface area is an intermediate light gray anodic coating.

11. The process as defined in claim 8, said voltage being adjusted so that the anodic coating formed on said predetermined surface area is a dark gray anodic coating.

12. An anodizing electrolyte which consists essentially of about 2% to about 12% phosphoric acid, about 9% sulfuric acid, by weight, the remainder being water.

13. Ananodizing electrolyte which consists essentially of about 4% to about 8% phosphoric acid, about 15% to about 24% acetic acid, and about 8% to about 20% JOHN H. MACK, Primary Examiner.

T. TUFARIELLO, Assistant Examiner.

' to about 30% acetic acid, and about 7% to about 

1. THE METHOD OF ANODIZING A JPREDETERMINDED SURFACE AREA OF AN ALUMINUM PART, WHICH COMPRISES APPLYING A LIQUID ELECTROLYTE TO A POROUS JPAD, SAID JPOROUS PAD BEING POSITIONED ON A METAL ELECTRODE, SAID ELECTROLYTE COMPRISING ABOUT 2 ABOUT 12% BY WEIGHT OF PHOSPHORIC ACID, ABOUT 9 TO ABOUT 30% BY WEIGHT OF ACETIC ACID, AND ABOUT 7% TO ABOUT 60% BY WEIGHT OF SULFURIC ACID, CONNECTING SAID ELECTRODE, SAID PAD AND SAID JPART IN AN ELECTRIC CIRCUIT WITH SAID PART FORMING THE ANODE, CONTACTING SAID PAD WITH SAID SURFACE AREA OF SAID PART, MOVING SAID PAD OVER SAID PREDETERMINED SURFACE AREA OF SAID PAT AND MAINTAINING CONTINUOUS CONTACT BETWEEN SAID KPAD AND SAID SURFACE AREA WHILE PASSING DIRECT CURRENT THROUGH SAID CIRCUIT, AND CONTINUING SAID MOVEMENT OF SAID PAD AND SAID PASSAGE OF CURRENT FOR A PERIOD SUFFICIENT TO FORM AN ANODIC COATING ON SAID PREDETERMINED SURFACE AREA OF SAID PART. 